The Rankine cycle is a thermodynamic cycle composed of two isoentropic transformations and two isobars. Its purpose is to transform heat into work. It is the basis of the design of steam engines of any type.
This cycle is generally adopted mainly in thermoelectric plants for the production of electrical energy and uses water as a motor fluid, either in liquid form or in the form of steam or gas, with the so-called steam turbine. For this use, water is demineralized and degassed properly.
On the other hand, it is rapidly falling into disuse in the field of railway traction and marine propulsion, supplanted by the diesel engine and the electric motor. Instead, it is still indispensable for nuclear engine equipment (power plants, submarines and aircraft carriers).
The Rankine cycle can be:
Open Rankine cycle, that is, with the discharge of steam into the atmosphere (as was the case with the old steam locomotives, which had to transport, in addition to coal, also water).
Closed Rankine cycle, as in the case of thermoelectric plants, including combined cycle plants. It is possible to exploit the residual heat of steam condensation (cogeneration), even transporting it through an urban heating network.
The four processes in the Rankine cycle
Process 1–2: the working fluid is pumped from low to high pressure. Since the fluid is a liquid at this stage, the pump requires little input energy.
In other words, Process 1-2 involves isentropic compression in the pump, isentropic process.
Process 2–3: The high pressure liquid enters a boiler, where it is heated at constant pressure by an isobaric process by an external heat source to become a saturated dry steam. The required input energy can be easily calculated graphically, using an enthalpy-entropy graph (hs graph or Mollier diagram), or numerically, using steam tables.
In other words, Process 2-3 is the addition of heat at constant pressure in the boiler, an isobaric process.
Process 3–4: Saturated dry steam expands through a steam turbine, generating energy. From a thermodynamic point of view, this lowers the temperature and pressure of the steam, and some condensation can occur. The output in this process can be easily calculated using the graph or tables indicated above.
In other words, Process 3-4 is isentropic turbine expansion, isentropic process.
Process 4–1: wet steam enters a condenser, where it condenses at a constant pressure to become a saturated liquid.
In other words, Process 4-1 is rejection of heat at constant pressure in the condenser, isobaric process.
In an ideal Rankine cycle, the pump and turbine would be isentropic, that is, the pump and turbine would not generate entropy and, therefore, would maximize net labor production. Processes 1–2 and 3–4 would be represented by vertical lines in the T - S diagram and would look more like the Carnot cycle. The Rankine cycle shown here prevents the state of the working fluid from ending up in the superheated steam region after expansion in the steam turbine, which reduces the energy eliminated by the condensers.
The actual steam energy cycle differs from the ideal Rankine cycle due to irreversibilities in the inherent components caused by fluid friction and heat loss in the surroundings; the friction of the fluid causes pressure drops in the boiler, the condenser and the pipe between the components, and as a result the steam leaves the boiler at a lower pressure; Heat loss reduces net work production, therefore, the addition of steam heat to the boiler is required to maintain the same level of net work production.
Rankin organic cycle
An organic Rankin cycle or ORC is a Rankin process with an organic solvent such as propane, isobutane, isopentane or ammonia instead of steam. A turbine is often used for this. Because an organic solvent has a lower boiling point than water, the Rankin organic cycle allows energy to be extracted from a lower temperature to 100 ° C. Therefore, this cycle is particularly suitable for using residual heat.
Operating principle of the Rankin organic cycle
The operating principle of the Rankin organic cycle is the same as that of the Rankin cycle: the working fluid is pumped to a boiler where it evaporates, passes through an expansion device (turbine or other expander), and then through a condenser heat exchanger where it finally condenses again. In the ideal cycle described theoretical model of the engine, isentropic expansion and thermodynamic processes of evaporation and isobaric condensation. In a real cycle, the presence of irreversibility reduces the efficiency of the cycle. These irreversibilities occur mainly during expansion and in heat exchangers.
Irreversibilities during expansion: then only part of the energy is obtained and the pressure difference becomes useful work. The other part becomes heat and is lost. The efficiency of the expander is determined by comparison with an expansion by an isentropic process.
Irreversibilities in heat exchangers: the liquid takes a long and winding path that guarantees a good exchange of thermal energy, but ensures that the pressure drops, resulting in a lower amount of energy in the cycle. The temperature difference between the heat source / sump and the working fluid also generates exergy and reduces cycle times.
In the case of a "dry fluid", Rankin's organic cycle can be improved using a regenerator because the fluid does not reach the biphasic state at the end of the expansion, the temperature at this point will be higher than the condensation temperature. This higher temperature can be used to heat the liquid before it enters the evaporator. Therefore, a backflow heat exchanger is mounted between the expander outlet and the evaporator inlet. The required power of the heat source is reduced by half and efficiency is increased.